Source-to-Sea: The Transport and Storage of Organic Carbon and Iron from Forested Environments to (Fjord) Sediments

Abstract

Fjords are globally important carbon stores due to their ability to store carbon long-term. However, the source of organic carbon and the influence of catchment processes and land cover on sedimentary carbon storage is poorly understood. This impacts our ability to effectively protect and manage dynamic coastal environments. Globally, ~ 20 % of organic carbon (OC) stored in sediment is directly bound to reactive iron (FeR). In this study, I explore the role of OC-FeR associations on the preservation and storage of OC in catchment and sediments of a mid-latitude Scottish fjord, Loch Creran. On average 37 % of OC was bound to FeR in forested catchment soils, 41 % in freshwater sediments, and ~16 % in fjord sediments. Ferrihydrite was found to be the dominant species of iron throughout the catchment and sediments, indicating that some OC-FeR complexes may travel from source (upstream and surrounding terrestrial environments) to fjord sediments without extensive reworking. On average, 47 – 67 % of OC in fjord sediments originated from terrestrial sources, identified from Bayesian modelling, and this was further explored using n-alkanes, specific δ¹³C isotopic signatures and pine resins. Methyl dehydroabietate (pine resin) was able to capture the presence of coniferous material where n-alkane abundance and isotopes could not. Furthermore, biomarkers were stored effectively in fjord sediments over long time periods, showing little degradation of either proxy downcore. Through conducting a source to sea study and utilising biogeochemical information across vegetation, soils, rivers and sediments, I was able to create a better understanding of the mechanisms that promote lateral carbon movement and the chemical changes that carbon complexes can withstand. I demonstrate that in mid-latitude fjords OC-FeR complexes play an important role in biogeochemical cycling, OC storage in forest soils, and OC transport and storage from source towards sea.